A built-up flywheel of the kind referred to above is generally provided with a first flywheel coupled to an output shaft at the engine side, a second flywheel elastically coupled to the rear surface of the first flywheel which is spaced a distance from the second flywheel and a clutch covering body secured to an outer periphery of the rear surface of the second flywheel.
The assignee of the present invention has already proposed some cooling mechanisms for cooling the above clutch covering body. For example, in U.S. Pat. No. 4,886,153, it is proposed that the lateral edge in the circumferential direction of a finger of a diaphragm spring used in the clutch covering body is notched to form wide slits and a vane member is provided in the slits to gather the cooling air.
Although the proposed cooling mechanism is able to cool the interior of the clutch cover effectively, it is not enough to cool the flywheel itself.
Meanwhile, a flywheel cooling mechanism proposed in Japanese Utility Model Laid-open Publication No. 48-43044 (43044/1973) has realized cooling of the flywheel. According to the proposal, a plurality of cooling fins are provided inwards at the side of a pressure plate of the flywheel, thereby raking the air inside the clutch cover to a pressuring surface of the flywheel.
Since the cooling fins are at the side of the pressure plate of the flywheel in the foregoing cooling mechanism and, no allowable space is there for the cooling fins, the area of the surface for each fin cannot be large. Therefore, it is impossible to obtain sufficient driving force (raking force) for the cooling air.